1. Field of the Invention
The present invention relates generally to semiconductor fabrication, and more particularly to immersion lithography.
2. Description of Related Art
Semiconductor fabrication processes generally include an optical lithography step where light passes through a mask that includes a pattern. The pattern is focused by a lens and is projected onto the surface of a semiconductor wafer. The pattern may be a particular layer of an integrated circuit including transistor structures, contacts, conductors, and so forth. As feature sizes of devices continue to decrease, the resolution of the lithography process becomes more important. The resolution of the lithography process, in particular the thickness of the conductors and spaces between the conductors, is proportional to the wavelength of light used for patterning, and inversely proportional to the numerical aperture of the lens. Ideally, the resolution of the lithography process is improved when the wavelength is small and the numerical aperture is large.
However, as wavelengths decrease to less than 193 nanometers, the light becomes absorbed, rather than passed, by the fused silica or amorphous silicon dioxide lenses. As such, to maximize the resolution, the numerical aperture needs to be maximized. The numerical aperture may be characterized by the following equation:Numerical Aperture=n ·sin θ  Eq. 1where n is the refractive index of the medium between the lens and the wafer and θ is the maximum half-angle formed by the light focused on the wafer. In optical lithography, the light passes through air which has a refractive index of 1 and an overall numerical aperture between 0 and 1. Therefore, to maximize the numerical aperture and resolution of the lithography process, the medium in which the light passes through needs to increase.
Immersion lithography is a technique in which lithographic exposure is performed with an immersion fluid, such as purified water, introduced between the lens and wafer. Typically, a small, planar layer of water that covers the section of the wafer to be exposed is introduced through an opening of a housing containing a planar lens element. After exposure, the water is suctioned out of the housing by a vacuum. By using immersion fluid, and in particular, water, as a medium between the light source and the wafer, the numerical aperture may be increased by a factor of up to 1.44 (the refractive index of water). However, a planar final lens surface in immersion lithography limits the design for a higher numerical aperture. As such, there is a need for a higher numerical aperture lens element.
The referenced shortcomings are not intended to be exhaustive, but rather are among many that tend to impair the effectiveness of previously known techniques concerning immersion lithography; however, those mentioned here are sufficient to demonstrate that the methodologies appearing in the art have not been altogether satisfactory and that a significant need exists for the techniques described and claimed in this disclosure.